dna and trace evidence

DNA

The building blocks of life, present in every cell nucleus; no two DNA samples are the same.

DNA ‘fingerprint’

A technique discovered by Sir Alec Jeffreys in 1984 that distinguishes every individual in the world.

Double Helix

The structure of nuclear DNA formed by two strands twisting around each other.

What are the specific base pairings in DNA?

adenine pairs with Thymine (A-T) and Guanine pairs with Cytosine (G-C)

Restriction Fragment Length Polymorphism (RFLP)

Variations in DNA fragment lengths with relatively long repeating DNA strands that form the basis for early DNA typing procedures.

Polymerase Chain Reaction (PCR)

A technique that offers a distinct advantage by amplifying minute quantities of DNA millions of times, allowing for analysis.

Short Tandem Repeats (STRs)

The most successful and widely used DNA profiling procedure that is less susceptible to degradation may often be recovered from bodies or stains that have been subjected to extreme decomposition 

• combines both RFLP and PCR

• works on small stains but creates high statistics 

Mitochondrial DNA Testing (mtDNA)

A testing method that can identify individuals of the same maternal lineage.

Combined DNA Index System (CODIS)

The FBI's software used for matching DNA profiles in databases. the success of the CODIS program is measured by the crimes it helps to solve 

National DNA Index System (NDIS)

The collective database of DNA profiles of the United States.

Touch DNA

A type of DNA collected from the most prolific human cells, often leaving epithelial cells.

Dna Probative Value

Evidence indicating that DNA belonging to a victim or suspect is found where it does NOT belong

Seminal Fluid Evidence Collection

Use of Alternative Light Source (ALS) to locate and presumptive test for possible seminal fluid.

Evidence Collection Procedures

Collect the entire item when possible, particularly clothing and change gloves before handling new evidence.

Locard's Theory of Exchange

The principle stating that the intensity, duration, and nature of contact determines material transfer. cross exchange bears as a silent witness

Hairs and Fibers Analysis

Investigators analyze color, length, and diameter to compare strands of human hair. The most common request is to determine whether or not hair recovered at the crime scene compares to hair removed from the suspect

Mechanical Fit

Individualization achieved by fitting two or more pieces of an object together.

Tool Mark Evidence

Marks left by tool use, providing class characteristics but also potentially unique imperfections.

Bite Mark Impressions

• ante mortem- less distinction. photograph for several days 

• post mortem- well defined, no bruising

the two forms of footwear evidence are…

• impressions 

• prints- may not be able to physically recover

Soil Evidence

Soil's value rests in its prevalence and transferability at crime scenes, requiring collection from multiple locations.

What are the four findings that can be related to the examination of bite marks

• definite – to the exclusion of all others (i.e. only the subject could have made the bite mark impression.)

• Consistent – no features present will exclude the suspect

• Possible– due to the nature of the injury, unable to positively confirm or exclude the suspect

• Exclusionary – definitely not made by the suspect

Impression Evidence- Tire 

remove the vehicle from the scene and transport to a secure facility for processing

Describe Paint as evidence in a crime scene

• One of the most common types of paint examined in the crime laboratory involves finishes emanating from automobiles

• these coatings may include electro coat, primer, primer, surfacer, basecoat, and clearcoat

How do you collect and preserve hair?

• 25 full length hairs from all areas of the scalp will normally ensure a representative sampling

• 25 full length pubic hairs should cover the range of characteristics present in pubic hair

• The investigators task of looking for minute strands of fibers often becomes one of identifying and preserving potential “carriers” of fiber evidence

• relevant articles of clothing should be packaged carefully in separate paper bags

• It is necessary to remove a fiber from an object, the investigator must use clean forceps, place it in a small sheet of paper, fold and label the paper, and place the paper packet inside another container

How do you minimize contamination at a crime scene?

Disposable forceps are to be used so that they can be discarded after a single evidence collection. If not disposable, clean with bleach.

The packaging of biological evidence in plastic or airtight containers…

must be avoided

What are the different types of evidence collection kits?

• SANE kit - Sexual Assault Nurse Exam

• suspect rape kit

• buccal swab collection kit